It is well known that if optical fibers are unevenly stressed, substantially losses can occur. Accordingly, it is advantageous to prevent stressing of the optical fibers.
It is suggested in U.S. Pat. No. 3,984,172 that stressing the optical fiber can be prevented by utilizing a "loosely coupled" structure between the optical fibers and their sheath to allow the fibers some lateral and longitudinal movement so as to prevent stressing them.
Another method to prevent stressing the optical fibers is to embed them between two thermoplastic films in such a manner that the fibers follow an undulate path. The undulate path allows the ribbon to be elongated. Such a method is illustrated in U.S. Pat. No. 3,937,559.
It is also well known in the prior art to embed optical bundles in an encapsulating medium along with strengthening members of a larger diameter such as shown in U.S. Pat. No. 3,887,265. In this patent, however, the encapsulating medium will necessarily cause the optical bundle to be mechanically coupled to the strengthening members.
In the prior art, it is well known to manufacture an optical cable by spirally wrapping a ribbon of optical fibers around a core and covering the resulting structure with a protective sheath. Typical such optical cables are shown in U.S. Pat. Nos. 3,883,218 and 3,937,559.
One of the main problems with prior art optical ribbons is their inability to withstand both elongating stresses and impacts without damaging the optical fibers. It is desirable to make optical cables on the same machinery used to manufacture electrical cables. However, this is a relatively difficult objective to achieve since the apparatus used to make electric cables is relatively rough in handling the strands which comprise the cable and the optical fibers by themselves are not able to withstand the strains which can be withstood by copper conductors. Further, because of the inability of prior art optical ribbons to withstand abuse imposed upon it by cable making machinery, the quality of prior art optical cables is less than desired.